Conotoxins are small, disulfide-rich peptides having the ability to differentiate between various types of ion channels, making them ideal diagnostic tools in the characterization of neuronal pathways and in drug development. One member of this family, ?-conotoxin MVIIA (Ziconitide) was recently approved by the FDA for the treatment of chronic pain. Evaluation of the role of disulfide bridges is essential in our understanding of protein folding. This proposal will explore how slight changes (introducing conformational restrictions or altering the size of the intramolecular bridges) in the sequences of small peptides affect folding properties. The target peptides are a-conotoxins and their analogues;thirteen or fourteen amino acid-containing peptide amides having four cysteine residues. The four cysteines can form two disulfide bridges, leading to three possible regioisomers, only one of which is found in nature. The amino acid proline's cyclic nature may impose steric constraints for the folding of the peptides, resulting in much better selectivity for the synthesis of the target native regioisomers. The work will focus in three areas: (i) Study of the variation in folding due to changes in the amino acid sequence and synthesis and biophysical characterization of conotoxins that contain a novel amino acid, proline-like with a basic side chain, which has been proven to be essential for increased biological activity. (ii) Synthesis and biophysical characterization of structures with homologues of cysteine or with a lactam bridge. And (iii) modeling (molecular dynamics and ab initio) of the molecular parameters, (S?S distance, rotational conformation of the side chains), relevant to activation energy of the bond formation that results in a specific regioisomer. Conotoxins are small peptides having the ability to differentiate between various types of ion channels, making them ideal diagnostic tools in the characterization of nervous system pathways and in drug development. For instance, Ziconitide (?-conotoxin MVIIA) has already been approved by the FDA for cancer and AIDS patients for the treatment of chronic pain. This proposal aims to develop methods to make contoxins more efficiently.